MFD_TC6393XB is ARM-only

Compile error on other architectures:

    CC      drivers/mfd/tc6393xb.o
  /home/bunk/linux/kernel-2.6/git/linux-2.6/drivers/mfd/tc6393xb.c: In function ‘tc6393xb_attach_irq’:
  /home/bunk/linux/kernel-2.6/git/linux-2.6/drivers/mfd/tc6393xb.c:324: error: implicit declaration of function ‘set_irq_flags’
  ...

Reported-by: Adrian Bunk <bunk@kernel.org>
Signed-off-by: Adrian Bunk <bunk@kernel.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>